METAL SALT NPS

Physicochemical properties of membrane can be enhanced by introducing metal salt NPs in the membrane structure (membrane matrix) or by coating NPs on the membrane surface. It is reported that distinct improvements on the performance of membranes, including salts separation, flux and antifouling properties are achieved.

Silver NPs

Several metal salts of N Ps are used for membrane modification, such as salts of silver and copper, which have exhibited antimicrobial properties. Silver NPs in the range lnm -100 nm have good characteristics, including; good conductivity, high mechanical properties, chemical stability and antimicrobial properties. They also have the ability to adsorb oxygen on its surface and easily interact with sulfur and phosphorous groups in bacterial proteins. Thus, silver NPs have vital role in enhancing membrane antifouling properties (Garcia-Ivars et al., 2019).

Preparation

Two methods including ex-situ and in situ reductions of silver have been reported. In the first method, silver nitrate (AgN0₃) is added to a solvent as a reducing agent for silver. The solution is heated under intense stirring to allow for NP formation. The formed NPs are then added to the membrane dope solution containing polymer, additives and solvent. The second method includes dissolving silver nitrate in a solvent at room temperature. The dope solution is then subjected to intense stirring under heating, and when it becomes homogeneous, the salt/solvent solution is added to initiate the reduction of silver ions to form the NPs (Taurozzi et al., 2008).

Effect of Silver NPs on Membrane Characteristics and Performance

Several studies reported the preparation of membrane NPs using silver NPs by blending or forming a coating of silver particles on the membrane surface in addition to its effect on membrane characteristics and performance. Toroghi et al. (2014) used two methods for membrane modification. The first method is coating PES membrane surface by soaking membrane in silver Nanoparticles solution where the solution is prepared by dissolving 1 g of fructose and 0.1 g of ammonium hydrogen citrate in 1 liter of distilled water and pH=9.5 and heated to 80° C, then 9.35 ml of 0.1 M silver nitrate solution was added to form silver NPs. The second method includes blending of NPs with membrane polymer solution

Effects of metal oxide NPs on the surface and performance of composite membranes

TABLE 2.5

through dissolving AgN0₃ in dimethylformamide (DMF) and stirred at 100°C for 1 h then, from 2 to 6 wt % of silver-DMF organosol was added to the casting solution containing PES and polyvinylpyrrolidone (PVP) and dimethylacetamide (DMAc). The results showed that the amount of silver formation in the coating method was much greater than in blending method. Both methods showed good antibacterial activity against E. coli and S. aureus. The first method exhibits significant changes in morphology or permeate, while the second method showed different cross sections, in addition to decrease of flux .

Hoek et al. (2011) prepared mixed matrix membranes by dissolving silver metal powder in n-methyl-2-pyrrolidone (NMP), followed by the addition of polysulfone. Introducing silver particles led to pore size increase from 14 to 25 nm, increased roughness, and decreased, the contact angle, while tensile strength increased from 26.8±0.6 to 48.1 ±2.8 MPa. Dolina et al. (2018) developed a method for modification by stabilizing silver NPs on hollow-fiber poly- ethersulfone membranes through soaking the membrane in a solution of silver nitrate for 4 h at room temperature. The modification results showed an increase in permeability of 15% when compared to the unmodified membrane, no changes in separation properties, decrease contact angle, and no changes in the original membrane’s cross-section morphology. Chou et al. (2005) studied the membrane matrix fabrication through the addition of cellulose acetate (CA) powder and 0.001-0.1 wt.. % AgN0₃ silver nitrate pellets to DMF solvent to form the dope solution. The modification results showed increase in permeability from 5.586 to 6.15 LMH .atm, dense structure in the sublayer and no apparent pore changes on the outer and inner surfaces. The mechanical properties showed a slight decrease in tensile strength and elongation. Li et al. (2013) prepared membrane mixed matrix through the in situ method in which 0.79 wt.. % of AgN0₃ was dissolved in DMF at room temperature and added to a homogenous membrane polymer solution containing PVDF, PVP and DMF and stirred at 50°C for a further 7 h. Introducing silver NPs led to increased pore size from 0.209 to 0.251 pm and to increased porosity. On the other hand, contact angle was reduced from 81° to 68°, while flux increased from 36.4 to 108.6 LMH with a decrease in rejection.

Copper (Cu) NPs

Copper NPs are solid particles; like other NPs, their size range is between lnm and 200 nm. Cu NPs possess excellent antimicrobial properties, are of low cost, and are more available than other NPs (Zahid et al., 2018).

Many techniques have been used to prepare nanocomposite membranes containing Cu NPs for improving antifouling properties of the membrane surface. Xu et al. (2015) modified polyacrylonitrile (PAN) ultrafiltration membrane surfaces by immersing membranes in the solution contained polyethylenimine (PEI) and copper sulfate (CuS0₄) powder to obtain a poly-cation-copper(II) complex on the membrane surface, followed by cross-linking. The developed membrane antibacterial efficiency was enhanced more than 95%. Membrane micropores were more compact than in pristine membranes.

Ben-Sasson et al. (2016) developed in situ formations of Cu NPs that coated the surface of TFC-RO membranes using copper salt and reducing agent. The modification showed reduction of biofouling, minor increase in flux, and slight decrease in NaCl rejection. Zhang et al. (2017) modified PA-thin-film composite membranes by coating Cu NPs membrane surfaces with copper chloride (CuCl₂) aqueous solution. The modified membrane showed an enhancement in antibacterial properties, with more than 99% efficiency, but flux decreased after fouling. An overview of the effect of silver and copper metal salt NPs on membrane characteristics and performance is given in Table 2.6.

NANOCELLULOSE ADDITIVE

Nanocellulose (NC) based materials are carbon-neutral, sustainable, recyclable, and nontoxic. Cellulosic materials can be converted into cellulose nanofibers (CNFs), nanowhiskers (CNWs), and nanocrystals (CNCs) with many potential applications (Missoum et al., 2013, Mariano et al., 2014, Carpenter et al., 2015).

Preparation

NC mainly consists of chemical CNCs or mechanically extracted (nanofibril- lated) NPs. First, pure cellulose products are pretreated to remove ash, wax, lignins, hemicellulose, and other noncellulosic compounds.

CNFs from precursors are prepared mechanically using ultrasonication, high-pressure homogenization, grinding/crushing, and microfluidization. CNWs, composed of short fibrils; are produced by acid hydrolysis. Cellulose Nanocrystals CNCs are high-aspect-ratio NPs produced via the acid hydrolysis of CNFs as shown in Figure 2.5 (Gopakumar et al., 2019)

Effect on Mechanicaf Properties

A typical stress - strain curve is given in Figure 2.6. The tensile strength and modulus decreased with the addition of CNCs, which is unexpected (Nair et al. (2017). This was attributed to the aggregation of CNCs, which causes defects rather than reinforcements. In contrast, the addition of CNCs increased the strain-at-break, indicating that the addition of CNCs and introduction of porosity has a positive impact on the strain and toughness of the electrospun membranes. The tensile strength and modulus decreased with the addition of CNCs, which is also unexpected.

Conversely, Goetz et al. (2016) reported that the tensile strength and E-modulus increased from 1.43 MPa to 3.31 MPa, 0.34 GPa to 1.16 GPa, respectively by addition of ChNCs with cellulose acetate. ChNC has positively influenced the tensile which can be attributed to the stiffening effect of the ChNCs coated on individual electrospun fibers, as well as the mats However, the strain was decreased, which is attributable to the restricted slippage of the electrospun fibers past each other owing to the tied junction points.

Effect of metal salt NPs on the characteristics and performance of composite membranes

FIGURE 2.5 Acid hydrolysis of cellulose for the production of CNCs. Permission from Elsevier (Gopakumar et al.. 2019).

FIGURE 2.6 Stress - strain curve of electrospun membranes. Permission from Elsevier (Nair et al. (2017).

Effect on Morphology

On increasing the concentration of CNCs with CA dissolved in DCM: A or (AA: A) solvents, the average size of the added CNC was reduced with no gain in the surface area (Nair et al. (2017). This can be explained by the increase in the conductivity of the solvent by the addition of CNCs, which is consistent with the findings of Naseri et al. (2015). Smith et al. (2019) studied the addition of CNCs and TOCNs in the polyamide thin film composite membrane and produced membrane with slight decrease in the average thickness. This decrease occurred as a result of slight differences in the interfacial polymerization conditions during sample preparation. Surface roughness decreased for the membranes fabricated via the dispersion method (addition CNC), while the roughness increased for the vacuum-filtered membrane (TONC). The presence of CNCs may have affected the surface formation of the polymer, but it is more likely that these variations result from small changes in interfacial polymerization reaction conditions. The surface zeta potential was improved for CA-CNC nanocomposite fibers, compared to CA fibers (Nair et al. (2017). Goetz et al.

(2016) used CA polymer-based electrospun mats to support Chitin nanocrystals (ChNC) to prepare microfiltration membranes with a hydrophilic surface . The average pore diameter decreased from 11.02 to 10.07 nm and surface area increased from 2.73 to 3.709 m2/g. Typical SEM and AFM images are given in Figures 2.7 and 2.8.

FIGURE 2.7 SEM image of fibers (a) CA(AA:A); (b)CA+CNC(AA:A); (c) CA(DCM: A); and (d) CA+CNC(DCM:A). Permission from Elsevier (Nair et al. (2017).

FIGURE 2.8 AFM image of porous fibers and particles d) 3D height image of CA and CA-CNC porous particle. Permission from Elsevier (Nair et al. (2017)).

Effect on Performance

Nanocellulose in a native form or with additional surface functionalization (tempo oxidation, enzymatic phosphorylation, cationization, etc.) have significant adsorption efficiency toward metal ions, nitrates, dyes, humic acid from industry effluents (Liu et al., 2015; Sehaqui et al., 2014).

Ma et al. (2012) studied the removal of crystal violet dyes using nanofi- brous microfiltration membranes based on CNWs by adsorption. They found that the maximum adsorption capacity of the CNW-based microfiltration membrane for crystal violet dye was higher than that of GSO-22 (a commercially available membrane). This was because of the strong electrostatic attraction between protonated of crystal violet dye molecules and the highly negative carboxylate group on the cellulose Nanowhiskers’ surface. These Nanofibrous microfiltration(MF) membrane system had high flux, low pressure drop, and a high retention capability against both bacteria and bacteriophages. Other researchers (Karim et al., 2014) showed that nanocrystals-based membranes prepared by freeze drying resulted in low water flux (64 LMH. MPa'¹), despite good functionality. This was attributed to the adoption of the freeze-drying method, which reduces H-bonding between nanocrystals and the relatively high thickness (250 p m). These membranes had a pore diameter of 13-17 nm and had the ability to immobilize positively-charged dye molecules with reported 84%, 69% and 98% removal rates for methyl violet 2B. rhoda- mine 6G, and Victoria Blue 2B. respectively. Partially hydrolyzed polyacryl- amide/cellulose nanocrystal (HPAM/CNC) Nanocomposites were investigated for the uptake of methylene blue (Zhou et al., 2014). The adsorption capacity of the nanocomposite increased with increasing CNC concentration to 20% and decreasing pH. A maximum capacity of 224.8 mg/g was detected for the nanocomposite composed of 20% CNC at pH 6.5r Recently, Gopakumar et al. (2017) removed toxic crystal violet dyes from water by using a cellulose nanofiber-based PVDF membrane with adsorption capacity 2.948 mg/g .This can be attributed to the electrostatic interaction between the protonated crystal violet and CNF-based PVDF membrane. Table 2.7 summarizes some of the reported data concerning the effect of nanocellulose materials additive on membrane characteristics and performance.